Not Applicable
Not Applicable
Field of the Invention
Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
The invention relates to a safety switching system for a clamping device, provided with clamping wedges and operable by a flowing medium for holding and/or lowering or raising a pipe string with which a borehole is lined in order to support the rock and create a transport line.
When creating a pipe string and lowering it into a borehole, clamping devices absorbing the considerable inertia forces of the pipe string are alternately used. One of the devices is a holding apparatus (spider), with which a pipe string is held with the help of clamping wedges for screwing to another pipe. For such a holding apparatus, according to U.S. Pat. No. 2,545,177, an apparatus for automatic control of the placing of the clamping wedges is used when the desired position of the pipe string has been reached.
The fitting of an individual pipe onto a pipe string takes place firstly with an auxiliary elevator, which transfers the pipe from a pipe store, places it onto the pipe string and holds it there during the screwing process. After the end of the screwing to the pipe string positioned under the fitted-on pipe, the auxiliary elevator is removed and replaced by the second clamping device, an elevator. This is essentially constructed identically to the holding apparatus and is pushed from above over the initially exposed pipe which is already screwed to the pipe string, until its clamping wedges reach the prescribed position on the pipe. Its clamping wedges are then put in place.
In practice, reaching the correct position on the pipe is difficult. The elevator does not allow a precise observation for determining the correct position of its clamping wedges. For example, gripping part of a pipe sleeve can lead to a slip during the assumption of the very high inertia forces and to considerable damage to the pipe sleeve, as it is to be taken into account that the elevator must hold the entire pipe string at the top end of a screwed-on pipe and lower it as soon as the holding apparatus (spider) is released from the pipe string. This is achieved by the elevator firstly raising the entire pipe string somewhat, to relieve the holding apparatus. Only then can its clamping wedges be released, for example with the help of compressed air. Furthermore, a misunderstanding is to be prevented which causes the holding apparatus (spider) to be opened, although the elevator is not yet closed, and a pipe held by the spider or even a longer pipe string to thereby be lost.
The loss of a pipe string through incorrect use of the elevator or a mis-opening of the spider entails high costs and must therefore be avoided. Therefore, with a safety apparatus according to DE 42 27 645 C2, a possibility was created of placing the clamping wedges of an elevator on the pipe as soon as a pipe sleeve enters the range of a contact switch. This switching device is mounted on the upper side of the elevator. Mechanical influences which can lead to a defect are not to be ruled out however and threaten the necessary high switching security.
The object of the invention is to create a safety switching system of the sort mentioned at the beginning which is arranged protected inside a clamping device, is not subjected to outside mechanical influences and rules out faulty switching with certainty. The invention for achieving this objective is distinguished by the fact that there is attached to the clamping wedge holder of the clamping device a trip cam for a safety switching valve, which is arranged at the basic body of the clamping device and is operable by the trip cam which is moveable with the clamping wedge holder when the clamping wedges assume their prescribed clamping position on the pipe string.
Preferably, the trip cam has a run-on ramp, a run-off ramp and a vertical switching surface located between these and can be traversed by a contact wheel of the switching valve.
The length of the switching surface is determined by the differences in diameter between the largest and the smallest of the pipes to be clamped as prescribed with the clamping device.
For use on an oil-rig using two alternately operable clamping devices (spider and elevator) it is proposed that the safety valve of one device is placed upstream of the control valve for the actuation of the working cylinder for clamping wedges of the other device.
Additionally, a short-circuiting switch is provided for every clamping device to bypass the safety switch. This switch is actuated to be able to introduce a clamping action at the first pipe of a pipe string to be formed.
The use of additional safety valves which reciprocally stop the control cylinders for the opening and closing of the clamping devices becoming active at the same time and accordingly prevent a simultaneous opening of both clamping devices, for example in the case of simultaneously faulty switching, is of further considerable significance as regards safety.
The individual components of the safety switching system are of extraordinary simplicity, developed in a manner customary in the market, and housed in a clamping device so that mechanical defects cannot arise from outside influences. Faulty switchings are ruled out with a high degree of certainty.